1. Field of the Invention
The present invention relates to variable capacitance components formed in a semiconductor substrate.
2. Discussion of the Related Art
A component having a capacitance varying in accordance with a voltage reference is used, for example, in a voltage-controlled oscillator (VCO).
FIG. 1 partially and schematically illustrates a perspective view of a first electrode of a variable capacitance formed in a lightly-doped N-type silicon substrate 1. Substrate 1 comprises a series of identical regularly spaced ribs 2 each having a rectangular planar base.
FIG. 2 illustrates, in a partial simplified lateral cross-section view, a Schottky diode using the first electrode of FIG. 1. The silicon surface of the first electrode may be completed by a heavily-doped N-type surface region 3. Region 3 is continuous and extends in the upper planar surface of ribs 2, along their vertical walls and in the planar surfaces of substrate 1 separating the bases of neighboring ribs. A layer 4 of a conductive material capable of forming a Schottky barrier, such as a metal silicide, covers the entire structure. The Schottky diode thus comprises a first electrode (cathode) formed by substrate 1 comprising ribs 2 and a second electrode (anode) formed by silicide 4. The diode is generally completed by metallizations (not shown) contacting, at their rear surface, substrate 1 and, at their front surface, silicide 4.
In a reverse biasing (positive voltage applied on the cathode), the diode of FIG. 2 behaves as a capacitor, the inter-electrode insulator of which is the space charge area (or depleted area) which extends in region 3, if present, and substrate 1.
FIG. 3 is a graph partially and schematically illustrating variation C(V) of capacitance C of the diode of FIG. 2 versus value V of the reverse voltage. Capacitance C drops when value V increases. From a given limiting value or pinch-off voltage Vp, capacitance C abruptly varies towards a low value Vth. Pinch-off voltage Vp is the value for which the extension of the depleted area in substrate 1 is such that the volume of ribs 2 is completely depleted by the carriers. Value Vth corresponds to the limiting value of extension of the depleted area into substrate 1.
A disadvantage of a variable capacitance such as previously described is the discontinuity in the capacitance variation around pinch-off voltage Vp. Due to this discontinuity, the capacitance (frequency) range corresponding to a reverse biasing beyond pinch-off voltage Vp has to be, in practice, excluded from the operation, which does not enable sweeping a wide frequency range in an application to a VCO.
For a VCO to operate over a wide frequency range, several distinct devices must thus be used to enable frequency tunings on different frequency ranges. This goes against the desire to reduce the size of devices.
The problems previously discussed for a Schottky diode also arise if a reverse-biased PN junction is used as a variable capacitance. As compared to the structure of the Schottky diode of FIG. 2, a very heavily-doped P-type surface region is formed in heavily-doped N-type region 3. The presence of the P-type region further increases the complexity of the setting of the doping of N-type region 3, if present. It should be reminded that region 3 is optional; the heavily-doped P-type region may thus replace it.
The previously-discussed problems generally arise in any variable capacitance.